Ammonia is generally produced by reacting hydrogen and nitrogen, according to the following reaction equation:3H2+N2→2NH3 
The H2 is generally obtained from synthesis gas (normally known as “syngas”), which in turn is obtained from a hydrocarbon feed material, which is subjected to steam reforming so as to produce a mixture comprising carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2), usually followed by a water gas shift reaction wherein carbon monoxide reacts with water so as to form carbon dioxide and hydrogen. After removal of CO2 (or otherwise separating H2 from the gas mixture), the hydrogen is available for reaction with nitrogen (N2). The latter is either present in the original gas mixture (as it is inert with respect to all steps preceding the ammonia synthesis conditions), or added later if obtained from air, in a unit separating nitrogen from oxygen. The hydrogen and nitrogen are subjected to compression and conversion into ammonia in a synthesis reactor.
Ammonia is frequently used as a starting material in the synthesis of urea. Urea (NH2CONH2) can be produced from ammonia and carbon dioxide at an elevated temperature of, typically, between 150° C. and 250° C. and an elevated pressure of, typically, between 12 and 40 MPa, in the synthesis zone of a urea plant. In this synthesis, two consecutive reaction steps can be considered to take place. In the first step ammonium carbamate is formed, and in the next step, this ammonium carbamate is dehydrated so as to give urea:2NH3+CO2→H2N—CO—ONH4  (i)H2N—CO—ONH4H2N—CO—NH2+H2O  (ii)
A reference process, shown in FIG. 1, for producing ammonia comprises a steam reforming process for producing hydrogen followed by reaction of said hydrogen with nitrogen produced in an air separation unit (ASU). A disadvantage of this process however is that significant energy is used to separate the air into nitrogen and oxygen but no use is made of the oxygen so produced.
Another reference process, such a shown in U.S. Pat. No. 6,448,441, which is incorporated herein by reference, involves the use of two parallel gasifiers, working at different operating conditions, in order to increase the CO2 rate for urea production when a natural gas gasifier is used to produce syngas. By using two gasifiers, it is possible to obtain the correct stoichiometry in the reaction mixture for subsequent production of ammonia. In the process of U.S. Pat. No. 6,448,441, there is a need to produce additional CO2 to obtain the correct stoichiometry for the reaction of ammonia and CO2 to nitrogen. This requires the combustion of additional carbonaceous material, for example natural gas, which consumes more raw materials and energy.
In the production of ammonia, as well as in the production of urea, it is thus desired to be able to present the starting material in the desired stoichiometry, and it is desired to reduce energy and material costs as much as possible.